WEDNESDAY, June 25, 2014 (HealthDay News) -- Attention, binge TV watchers: New research suggests that long stretches spent glued to the tube may be more than just a guilty pleasure -- they could also shorten your life.

The study of more than 13,000 seemingly healthy adults in Spain found that those who spent more than three hours a day watching television had double the risk of early death compared to those who watched less than an hour a day.

"It is a little bit surprising," said study author Dr. Miguel Martinez-Gonzalez, a professor of preventive medicine at the University of Navarra, in Pamplona, Spain.

Gonzalez said the study participants were highly educated, and slim and active. Their average age was 37.

Anyone who said they had diabetes, heart disease or cancer was excluded at the start of the study.

Over the roughly eight years they were followed, there were 97 deaths -- 19 from heart disease, 46 from cancer, and 32 due to other causes such as accidents and liver or kidney disease.

Researchers then compared the risk of death with time spent in sedentary activities, such as watching TV, working at a computer and driving.

No link was found between early death and driving or working at a computer, according to the study, published online June 25 in the Journal of the American Heart Association.

But the risks associated with watching TV seemed striking. For every two additional hours a person spent watching TV, the risk of death from heart disease jumped by 44 percent, cancer death risk climbed by 21 percent and the risk of premature death went up by 55 percent for all other causes, compared to people who said they watched less than an hour each day.

All that just from watching TV? Gonzalez said even the researchers were skeptical about their findings.

They wondered, for example, whether people already gravely ill might watch more TV because they weren't physically able to do anything else. So they ran the numbers a second time, excluding the 35 deaths that happened in the first three years of the study, hoping to knock out people who might have been very sick. But the results only got stronger.

People tend to consume more processed foods and sugary drinks when they watch TV, so the researchers adjusted their results to control for the effects of added snacks and sweetened drinks. Again, the results got stronger.

They continued to try to eliminate things that might explain the results, such as smoking, age, sex, weight, and whether or not people followed a Mediterranean diet. Nothing seemed to really affect the results.

It's not the first study to find a link between TV time and premature death. But, the new research didn't prove that prolonged TV watching leads to an early death, it just found a connection between the two.

Experts not involved with the research said caution is warranted in interpreting the results.

"This is an interesting study, which raises interesting questions," said Dr. Lennert Veerman, a senior research fellow in public health at the University of Queensland, in Australia.

He said the link between TV viewing and early death is probably an indirect one. The more time people spend watching TV, the less time they have for things that are known to prolong life, such as seeing friends or exercising, he said.

And he thinks the current study still failed to account for some potentially important factors such as mental health, unemployment and alcohol use.

"I personally still think it is most likely that TV viewing was more a risk indicator than a cause of deaths," Veerman said.

But Gonzalez thinks time in front of the tube, alone, could be hazardous to a person's health.

"The time we spend watching television is a highly sedentary time," he said.

When driving or working at a computer, people at least move a little.

"You have tension in your muscles. You are moving little parts of your body, like your hands. You are not completely relaxed as you are when you are watching television," he said.

"For me, the major take-home message is to avoid spending many continuous hours watching television, to limit the time to one or at most two hours a day," Gonzalez said.